In the LCD, a so-called twisted nematic (TN) mode is widely used, which applies an electric field in a vertical direction with respect to a substrate having a liquid crystal layer. In this case, nematic liquid crystals are twisted and arranged between two polarization plates orthogonal to each other. According to this mode, the liquid crystals rise with respect to the substrate at the time of dark state, so that a birefringence occurs due to the liquid crystal molecules when seen in an aslant direction, which thus causes an optical leakage. To cope with this problem, a technique of optically compensating the liquid crystal cell using a film in which liquid crystalline molecules are hybrid-aligned, has been put into practice. However, it is very difficult to completely and optically compensate for the liquid crystal cell even when the liquid crystalline molecules are used, so that a gray inversion cannot be completely suppressed in a lower screen.
To deal with the problem, an LCD, using a so-called in-plane switching (IPS) mode of applying a traverse electric field to the liquid crystal or a vertically aligned (VA) mode of aligning liquid crystals with a negative dielectric anisotropy by means of protrusions or slit electrodes formed within panels, has been suggested and put into practice. Recently, these panels are used not only for a monitor but a TV, so that a luminance of the screen significantly increases. Accordingly, a few optical leakages in an inclined incident direction of an opposing angular location at the time of dark state, which was problematic in the conventional operation modes, have been recognized as a cause of degrading the display quality.
As one means for improving such hue or a viewing angle of the dark state, it is studied even in the IPS mode to dispose an optical compensation material having a birefringence property between a liquid crystal layer and a polarization plate. For example, a method of disposing birefringence medium of which optical axes having a function of compensating the increase and decrease of the retardation of the liquid crystal layer at the time of inclination between the substrate and the polarization plate to improve the color when the white color or the halftone display is directly viewed in the inclined direction (see JP-A-9-80424). In addition, methods of using an optical compensation film formed of a discotic liquid crystalline compound or a styrene polymer having a negative intrinsic birefringence (see JP-A-10-54982, JP-A-11-202323, and JP-A-9-292522), a method of combining a film of which a birefringence is positive and an optical axis lies in the in-plane of the film and a film of which a birefringence is positive and an optical axis lies in a direction normal to the film (see JP-A-11-133408), a method of using a biaxial optical compensation sheet having the retardation of one half wavelength (see JP-A-11-305217), and a method of preparing an optical compensation layer having a positive retardation on a surface of the film having a negative retardation as a protective film of the polarization plate are proposed (see JP-A-10-307291).
However, most of the proposed methods remove the birefringence anisotropy of the liquid crystal of the liquid crystal cell to improve the viewing angle, so that optical leakages due to deviation from the orthogonal angle between the polarization axes crossing each other when the orthogonal polarization plates are seen in the inclined direction cannot be sufficiently suppressed.
In addition, even when such optical leakages may be compensated, it is very difficult to completely and optically compensate for the liquid crystal cell without any problems. The major cause lies in the optical anisotropy of the protective film of the polarization plate, and in order to carry out the optical compensation including the same, the anisotropy of the retardation film should extremely increase, or an additional optical anisotropical layer should be given.
A method of decreasing the phase difference value of the protective film has been tried, which uses a film containing a norbornene resin or a resin having an imide group and a resin having a phenyl group and a nitrile group for the protective film (see JP-A-2004-4641 and JP-A-2004-4642). However, these synthesized resins are generally hydrophobic and are apt to be stripped due to a problem in adhesion with the polarization film. In addition, it cannot be expected that water vapors be transmitted from the polarization film in a process of stacking the protective film on both of the polarization films, so that the moistures remain inside to cause the degradation of the polarization performance.
An object of the present invention is to provide an IPS type LCD which has a simplified structure and a significantly improved viewing angle as well as display quality.